For at least the past 15 to 20 years, it has been known to incorporate recycled asphalt pavement (RAP) with various quantities of virgin aggregate material to produce a desirable and consistent blend for resurfacing roads. To produce such blends, the challenge for manufacturers has been to create a production unit that addresses the problems currently associated with the production of RAP, namely: (1) the generation of environmental pollutants; and (2) the production of by-products which adversely affect the life expectancy of the equipment used in the plants. In addition, it is always desirable to design the system so that a minimum amount of energy is required, and production costs are thereby minimized. The above concerns are particularly significant when RAP is added in percentages of 20% or more. For example, the by-products and pollutants created by combining cold RAP with superheated virgin aggregate causes hydrocarbons and steam which chokes the filtering system of the main plant which in turn wreaks havoc with pressures in the baghouse and static pressure in the combustion chamber resulting in lower production rates.
The superheating of virgin aggregate also creates a volatile situation in drum mix plants because as the higher percentages of recycle are being incorporated into the hot mix asphalt, the virgin aggregate material (VAM) temperature must be raised, often above the flash point of the liquid asphalt cement (AC), in order to reach the job specified mix temperature.
In batch plants the enormous amount of steam created by combining the RAP, the VAM and the liquid AC in the mill may cause an explosion which destroys the properties of the virgin liquid AC while creating hydrocarbons which cannot be recaptured and are thus released into the atmosphere.
Various artisans have attempted to address the problems associated with asphalt recycling facilities. For example, U.S. Pat. No. 4,600,379 to Elliott discloses a drum heating and mixing apparatus having two, concentric drums that heat aggregate material first in the inner drum and then in the space between the inner and outer drums. An exhaust gas outlet duct operatively connects the exhaust gas from the inner drum to the atmosphere, while an exhaust gas feedback siphons gas emitted in the space between the drums for incineration through the system burner. In the Elliott system only some of the hydrocarbons are returned to the drum drying burner so that the volume of steam and hydrocarbons produced by the system is not being totally captured. At higher rates of RAP therefore, the system, if not choked off, will vent into either the atmosphere or the filter house, potentially causing the filter house bags to clog.
U.S. Pat. No. 5,090,813 to McFarland and U.S. Pat. No. 5,201,839 to Swisher each include a step to take at least some of the moisture out of the RAP before it is added to the VAM. A parallel dryer is used with a portion of the area set aside for preheating the RAP. Total air for the system is supplied by one exhaust fan thus gases created by both dryers must pass through the filter house. The area set aside for preheating the RAP is approximately one quarter of the total area of both the RAP and mixing dryer and the counterflow aggregate dryer. Therefore chamber temperatures tend to reach undesirable heights because of the rate at which the RAP drying burner must operate in order to raise the temp of the RAP. Total moisture removed from the RAP cannot be established before introduction of the RAP to the VAM.
U.S. Pat. No. 5,251,976 to Milstead is directly related to RAP being introduced in the Hot Elevator of the batch type plant and a diversion chute booting into designated hot bin #1. The percentages are normally controlled in much the same way a blending control is used on the drum mix type plants. This type of operation limits the contractor of a batch plant to one type of mix at any one time. Unfortunately, both state and federal regulations prohibit this type of mixing method because the segregation of the RAP and VAM cannot be controlled. Also moisture from the RAP causes corrosion of the bins, screens, and the hot elevator in a short period of time.
U.S. Pat. No. 4,540,287 to Servas et al. discloses an apparatus which is used to combine RAP and VAM in a totally separate mixing drum with liquid AC and all hydrocarbons and steam being directed to the main plant burner. Job specified mix temperature is established after mixing has been completed. Problems that have arisen on this apparatus are the ignition or flaming of the mix while trying to achieve desired temperatures; and when attached to a batch plant, again, as with almost all the previously discussed designs, the versatility of changing different types of mixes throughout the day is very limited.
A recent survey regarding asphalt recycling conducted by Future Technology Surveys of Lilburn, Ga., determined that the asphalt manufacturing industry is particularly concerned with: (1) a simple technology which will result in one piece of equipment that meets industry standards for processing recycle for both batch plant and drum mix operations; and (2) operating costs in running recycle. The present invention addresses both of those concerns.